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setInterval</a></li><li><a href="/docs/js/note/array-deduplica.html" class="sidebar-link">数组去重</a></li><li><a href="/docs/js/note/download-file.html" class="sidebar-link">使用 JS 下载文件</a></li><li><a href="/docs/js/note/top-nav.html" class="sidebar-link">设置导航动态显示 / 隐藏</a></li><li><a href="/docs/js/note/anchor-transition.html" class="sidebar-link">JS 实现点击锚点过渡效果</a></li><li><a href="/docs/js/note/data-list.html" class="sidebar-link">数据处理</a></li></ul></section></li></ul> </div></div><div class="el-scrollbar__bar is-horizontal"><div class="el-scrollbar__thumb" style="width:0;transform:translateX(0%);ms-transform:translateX(0%);webkit-transform:translateX(0%);"></div></div><div class="el-scrollbar__bar is-vertical"><div class="el-scrollbar__thumb" style="height:0;transform:translateY(0%);ms-transform:translateY(0%);webkit-transform:translateY(0%);"></div></div></div></aside> <main class="page"> <div class="re-page-top"><h1 class="title"><span>DOM 操作跨线程</span></h1> <div class="re-page-index"><div class="page-index-title"><span class="open-catalog"><svg aria-hidden="true" class="icon arrow-right"><use xlink:href="#icon-bold-right"></use></svg> <span class="text">目录</span></span></div> <div class="page-index-content"><div class="page-catalog" style="display:none;"><div class="synopsis-wrap"><ul class="synopsis-ul"><li class="synopsis-li"><a href="/docs/js/base/dom-operate-across-threads.html#原理" class="synopsis-link">原理</a></li><li class="synopsis-li"><a href="/docs/js/base/dom-operate-across-threads.html#插入一个标签的完整过程" class="synopsis-link">插入一个标签的完整过程</a></li><li class="synopsis-li"><a href="/docs/js/base/dom-operate-across-threads.html#可能会触发重新渲染的操作" class="synopsis-link">可能会触发重新渲染的操作</a></li><li class="synopsis-li"><a href="/docs/js/base/dom-operate-across-threads.html#属性同步" class="synopsis-link">属性同步</a></li></ul></div></div></div></div></div> <div class="theme-default-content content__default"><h2 id="原理"><a href="#原理" class="header-anchor">#</a> 原理</h2> <p>JS 引擎是不能直接操作页面的，只能操作 JS，而渲染引擎又不能直接操作 JS，只能操作页面。JS 是想要通过 <code>document.body.appendChild(div)</code> 操作页面，就需要浏览器帮助，当浏览器发现 JS 在 body 中添加了一个 div，就会通知渲染引擎，新增一个与 JS 中 div 属性一模一样的 div。这就叫做<strong>跨线程通信</strong>。</p> <p>简单来说就是JS虽然不能直接操作页面，但是浏览器会帮你渲染页面。</p> <h2 id="插入一个标签的完整过程"><a href="#插入一个标签的完整过程" class="header-anchor">#</a> 插入一个标签的完整过程</h2> <ol><li><p>元素放入页面之前</p> <ul><li>你对元素所有操作都在 JS 线程中。</li></ul></li> <li><p>元素放入页面时</p> <ul><li>浏览器发现JS的意图，通知渲染引擎在页面中渲染对应的元素。</li></ul></li> <li><p>元素放入页面之后</p> <ul><li>你对元素的任何操作都有可能触发重新渲染。</li></ul></li></ol> <h2 id="可能会触发重新渲染的操作"><a href="#可能会触发重新渲染的操作" class="header-anchor">#</a> 可能会触发重新渲染的操作</h2> <p>由于浏览器对页面的重新渲染会很耗费内存，所以浏览器会想尽办法减少重新渲染的次数，在某些不必要的情况下并不会进行重新渲染。或者会尽量合并多次的 JS 操作进行重新渲染，以此减少重新渲染的次数。</p> <p><strong>1. 通过修改 <code>id</code> 属性触发重新渲染</strong></p> <p>比如一个获取到一个 div，修改它的 <code>div.id = 'newId'</code>。此时如果 newId 附带样式等操作，就会触发重新渲染，如果只是id，可能不会触发。</p> <p><strong>2. 通过修改 <code>title</code> 属性触发重新渲染</strong></p> <p>假如有如下样式的 div</p> <div class="language-html extra-class"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>div</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>div<span class="token punctuation">&quot;</span></span> <span class="token attr-name">title</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>titleHi<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
</code></pre></div><div class="language-css extra-class"><pre class="language-css"><code><span class="token selector">div::after</span> <span class="token punctuation">{</span>
    <span class="token property">content</span><span class="token punctuation">:</span> <span class="token function">attr</span><span class="token punctuation">(</span>title<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>运行以下代码</p> <div class="language-js extra-class"><pre class="language-js"><code>div<span class="token punctuation">.</span>title <span class="token operator">=</span> <span class="token string">'newId'</span>
</code></pre></div><div class="custom-block info"><p class="custom-block-title">原理</p><p>当它的 <code>title</code> 被修改时，由于 after 伪元素需要通过它的 <code>title</code> 属性来渲染，所以会触发浏览器的重新渲染。</p></div><p><strong>3. 通过获取宽度触发重新渲染</strong></p> <p>假如有如下样式的 div</p> <div class="language-html extra-class"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>div</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>test<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
</code></pre></div><div class="language-css extra-class"><pre class="language-css"><code><span class="token selector">.start</span> <span class="token punctuation">{</span>
    <span class="token property">border</span><span class="token punctuation">:</span> 1px solid red<span class="token punctuation">;</span>
    <span class="token property">width</span><span class="token punctuation">:</span> 100px<span class="token punctuation">;</span>
    <span class="token property">height</span><span class="token punctuation">:</span> 100px<span class="token punctuation">;</span>
    <span class="token property">transition</span><span class="token punctuation">:</span> width 1s<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token selector">.end</span> <span class="token punctuation">{</span>
    <span class="token property">width</span><span class="token punctuation">:</span> 200px<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>运行如下代码</p> <div class="language-js extra-class"><pre class="language-js"><code>test<span class="token punctuation">.</span>classList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">'start'</span><span class="token punctuation">)</span>
test<span class="token punctuation">.</span>clientWidth             <span class="token comment">// 这句话看似无用，实际会触发重新渲染</span>
test<span class="token punctuation">.</span>classList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">'end'</span><span class="token punctuation">)</span>
</code></pre></div><p>案例链接：<a href="http://js.jirengu.com/yefac/1/edit?html,css,js,output" target="_blank" rel="noopener noreferrer">http://js.jirengu.com/yefac/1/edit?html,css,js,output<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <div class="custom-block info"><p class="custom-block-title">原理</p><p>当你连续对同一个 div 做多次操作时，浏览器可能会合并为一次操作，在某些情况下，会导致动画的效果无法展现。这时可以获取一下元素的宽度，让浏览器不得不触发重新渲染（因为在获取宽度时必须要知道当前元素真正的宽度，也就是把它放进浏览器窗口中）。</p></div><h2 id="属性同步"><a href="#属性同步" class="header-anchor">#</a> 属性同步</h2> <ol><li>一个元素默认的标准属性被修改时，会自动同步至浏览器，比如 <code>id</code>、<code>className</code>、<code>title</code> 等。</li> <li>带有 <code>data-</code> 的属性原理同上。</li> <li>如果给元素随便赋值一个非标准属性，如 <code>x=&quot;1&quot;</code> 则对该元素的属性修改只会停留在 JS 中，并不会同步到页面。</li></ol> <div class="custom-block info"><p class="custom-block-title">结论</p><p>如果需要自定义一个属性，并希望它在修改时可以被浏览器同步到页面，就使用 <code>data-</code> 作为前缀。</p></div></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">上次更新：</span><span class="time">2021-02-10 12:44</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev"><svg aria-hidden="true" class="icon"><use xlink:href="#icon-left"></use></svg> <a href="/docs/js/base/node-list.html" class="prev">节点中的伪数组</a></span> <span class="next"><a href="/docs/js/base/create-jquery.html">jQuery 中的链式操作</a> <svg aria-hidden="true" class="icon"><use xlink:href="#icon-right"></use></svg></span></p></div> </main></div><div class="global-ui"><!----><!----></div></div>
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